1. Field of the Invention
The present invention relates to a wire electrical discharge machine that adjusts the amount of a working fluid based on the machining state.
2. Description of Related Art
In a wire electrical discharge machine, a voltage is applied to a machining gap between a wire electrode and a workpiece so that an electrical discharge occurs, and at the same time relative positions of the wire electrode with respect to the workpiece are changed with use of servo motors. Under such a condition, while a working fluid is being supplied to the machining gap, the workpiece is machined into a desired shape.
It is well-known that there is a correlation between a voltage (machining gap voltage) applied to the machining gap and a size (or length) of the machining gap. Therefore, controlling the machining gap voltage to be constant can lead to highly precise machining and it may result in the workpiece with a uniform machined surface.
However, the machining gap voltage might not represent an accurate size of the machining gap between the wire electrode and the workpiece when the machining state in the machining gap is unstable.
FIG. 1 illustrates a model of machining states in a machining gap. A voltage Vavg (machining gap voltage) applied to a machining gap between a wire electrode 1 and a workpiece 2 has the correlation shown in the following Expression 1 with respect to a size Td of the machining gap.Vavg∝Td  (1)
However, the machining gap voltage Vavg does not represent the accurate size of the machining gap between the wire electrode 1 and the workpiece 2 when the machining gap is in an unstable state, that is, in a state in which machining chips 20 are unevenly distributed. When a board thickness of the workpiece 2 is large, supply of the working fluid to the workpiece 2 is poor and moreover an influence of the vibration and flexure of the wire electrode 1 increases. This results in locally concentrated electrical discharges, and is considered the cause of the uneven distribution of the machining chips 20 as illustrated in FIG. 1. In this case, a machining gap voltage Va with a small amount of the machining chips 20, a machining gap voltage Vb with no machining chips 20, and a machining gap voltage Vc with a large amount of the machining chips 20 differ from each other, so that, under this condition, it is difficult to determine the accurate size Td of the machining gap between the wire electrode 1 and the workpiece 2 from the voltage Vavg (average value). Moreover, the machining gap voltage Vavg can be obtained by measuring a voltage between the wire electrode 1 and the workpiece 2. Well-known wire electrical discharge machines are usually equipped with means for measuring the machining gap voltage Vavg.
Factors that cause the unstable machining state include poor exhaustion of the machining chips 20 and poor supply of the working fluid. In general, the machining state tends to become unstable especially when the board thickness of the workpiece 2 is large because the working fluid is supplied to the workpiece 2 from both an upper working fluid feeder 40a and a lower working fluid feeder 40b. When the board thickness of the workpiece 2 is large, it is clear that the supply of the working fluid to the center of the workpiece is poor, and moreover the influence of the vibration and flexure of the wire electrode 1 increases. This results in locally concentrated electrical discharges. As a result, the machining chips 20 are considered to be unevenly distributed between the wire electrode 1 and the workpiece 2 in a direction in which the wire electrode 1 travels. Moreover, the wire electrode 1 travels from an upper wire guide 4a toward a lower wire guide 4b during the machining of the workpiece 2.
Japanese Patent Application Laid-Open (JP-A) No. 3-111121 and JP-A No. 7-60548 disclose a method of obtaining electrical discharge positions from current values or voltage values on conduction paths, and changing and controlling machining conditions, by recognizing a machining area from the distribution of the electrical discharge positions that are obtained. However, the method disclosed in these patent literatures attempts to avoid a concentrated electrical discharge attributable to uneven distribution of machining chips, in recognition of uneven distribution of discharge positions as the uneven distribution of machining chips. Therefore, there is a possibility that an excessive adjustment is carried out when the machining conditions are adjusted in a state in which the machining state has changed. For instance, in a state in which unmachined portions of the workpiece are unevenly distributed, the discharge positions are also unevenly distributed. However, there is no existing technology to determine whether the electrical discharge contributes to the machining of the workpiece. Furthermore, if the machining conditions are unnecessarily adjusted when normal electrical discharges are carried out, such an adjustment may impair normal electrical discharge machining.
JP-A No. 2010-280046 discloses the followings: the machining state is classified into “open”, “leak”, “electrical discharge”, “immediate electrical discharge”, and “short-circuit”; reference values which are set beforehand and state determination data are compared; and a feeding rate of wire, rest time, a flow rate of working fluid, and an application voltage are changed in accordance with the comparison result. This patent literature also discloses that supply of the working fluid is increased to promote exhaust of the machining chips because, when the state “leak” is dominant, it is highly assumed that the machining chips remain. In this technology, the exhaust of the remaining machining chips can be promoted by determining the machining state. However, the machining conditions can be more appropriately adjusted if the following is determined: at which part of the machining gap between the wire electrode and the workpiece, the determined machining state appears, and at which position the machining chips exist. Accordingly, regarding this technology, there is still room for improvement in the above-mentioned point of view.
JP-A No. 4-261713 discloses a method of independently controlling pumps that supply a working fluid to an upper nozzle and a lower nozzle, respectively so that a pressure value of the working fluid can follow a pressure value of the working fluid which has been instructed beforehand. In the method disclosed in this patent literature, in order to appropriately exhaust machining chips, it is necessary to recognize the uneven distribution of the machining chips and adjust the flow rate or the pressure of the working fluid.